The present invention generally relates to the production of printed images on fabric substrates, and more particularly to a specially-treated ink transfer sheet which is used to thermally deliver ink materials to a fabric substrate in a manner which produces a vivid and stabilized (e.g. waterfast) printed image.
In recent years, the popularity of xe2x80x9cpersonalizedxe2x80x9d printed clothing has greatly increased. For example, a variety of different techniques have been developed involving the production of custom-printed T-shirts and other clothing items. Of primary importance is the use of xe2x80x9ctransfer sheetsxe2x80x9d which contain monochrome (e.g. single color) or multi-colored printed images that are placed on a clothing item, followed by the application of heat thereto. As a result, the printed image on the sheet is xe2x80x9cheat transferredxe2x80x9d directly to the clothing item or other fabric substrate. This type of process along with representative ink transfer sheets and related procedures is discussed in a variety of references including U.S. Pat. Nos. 4,664,670; 4,758,952; 4,767,420; 4,980,224; 4,966,815; 5,139,917; and 5,236,801.
The basic ink transfer sheets of primary concern in the present case are commercially-available products which can be obtained from, for example, Foto-Wear, Inc. of Milford, Pa. (USA). These sheets normally involve three main layers, namely, (1) an inert backing layer which is ultimately removed and discarded; (2) a detachable release layer positioned on the backing layer which is designed for easy removal from the backing layer during the thermal transfer process; and (3) an ink receiving (e.g. ink absorbent) layer positioned on the release layer. In use, a printed image is initially applied to the ink receiving layer as discussed in greater detail below. Thereafter, the ink transfer sheet containing the printed image is positioned on a desired fabric substrate (e.g. a T-shirt or other clothing item) with the ink receiving layer (and printed image thereon) directly contacting the substrate. Heat is then applied by a conventional heated platen apparatus known in the art for thermal transfer purposes or a standard household iron in an amount sufficient to cause the release layer and accompanying ink receiving layer (containing the printed image) to adhere to the substrate. Because the release layer is typically produced from a low melting point polymeric composition, it softens substantially during the heating process which not only facilitates adhesion to the fabric substrate but also enables rapid detachment of the release layer from the backing layer. During or immediately after the application of heat to the ink transfer sheet on the fabric substrate, the backing layer is physically removed (e.g. peeled away) from the remaining layers of the transfer sheet. As a result, the release layer and attached ink receiving layer containing the printed image remain on the fabric substrate. In this manner, the printed image is effectively transferred to the substrate to generate a printed final product. It is important to note that the printed image (which is usually applied to the ink transfer sheet in a xe2x80x9creversexe2x80x9d configuration so that it will be properly oriented on the fabric substrate) is readily visible on the substrate since the release layer and ink receiving layer are substantially colorless (e.g. transparent). As a result, the printed image can be seen through these layers.
Heat-based ink transfer systems of the type described above have recently become available to consumers for in-home use. Consumers are now able to apply computer-generated or other images directly to a selected ink transfer sheet using commercially-available printing devices of conventional design. However, whether the printing process is being undertaken by consumers or on a large-scale commercial level, it is important that the printed image be stable or xe2x80x9cwaterfastxe2x80x9d after it is applied to a selected fabric substrate. The term xe2x80x9cwaterfastxe2x80x9d as used herein shall signify a printed image which does not smear, bleed, run, fade, or the like when exposed to moisture (e.g. water and/or water-based materials). If the printed image on the fabric substrate (e.g. T-shirt) is not sufficiently waterfast, it will progressively fade after repeated machine washings, thereby resulting in a product with a dull and indistinct character.
Prior to development of the present invention, a need existed for an effective thermal transfer process in which the resulting printed images remained clear, stable (e.g. waterfast), and fade-resistant over time. The present invention satisfies this goal through the use of a unique modified ink transfer sheet which includes chemical compositions that are capable of binding to charged coloring agents (e.g. dye molecules) in order to produce stabilized images. Likewise, the claimed invention is especially suitable for use in connection with thermal inkjet printing systems which enable the entire printing process to be accomplished by consumers at home. The claimed process and transfer sheets therefore represent an advance in the art of thermal transfer printing as discussed in greater detail below.
It is an object of the present invention to provide an improved ink transfer sheet and process for applying printed images to fabric substrates.
It is another object of the invention to provide an improved ink transfer sheet and process for applying printed images to fabric substrates which is suitable for use with many different printing systems.
It is another object of the invention to provide an improved ink transfer sheet and process for applying printed images to fabric substrates which is especially appropriate for use with inkjet printing systems (e.g. thermal inkjet units and other comparable systems).
It is another object of the invention to provide an improved ink transfer sheet and process for applying printed images to fabric substrates which uses a minimal number of process steps and materials to transfer the desired images.
It is a further object of the invention to provide an improved ink transfer sheet and process for applying printed images to fabric substrates in which the printed images are highly stable (e.g. waterfast) and fade resistant after repeated machine washings.
It is a further object of the invention to provide an improved ink transfer sheet and process for applying printed images to fabric substrates in which the printed images remain vivid and crisp after repeated machine washings.
It is a still further object of the invention to provide an improved ink transfer sheet and process for applying printed images to fabric substrates which is suitable for use in connection with a wide variety of different ink compositions and fabric substrates.
It is an even further object of the invention to provide an improved ink transfer sheet and process for applying printed images to fabric substrates which is readily applicable to both monochrome (e.g. single color) and multi-color printed images.
It is an even further object of the invention to provide an improved ink transfer sheet and process for applying printed images to fabric substrates which generally involves a minimal level of complexity and is suitable for use by both commercial users and consumers on an in-home basis.
In accordance with the present invention, a highly efficient method for applying clear, vivid, and stable printed images onto fabric substrate materials (e.g. T-shirts and other clothing items) is disclosed. The claimed ink transfer sheet and printing method enable the production of printed images using readily-available ink materials, with the resulting images being highly stable (e.g. waterfast) and fade-resistant even after multiple machine washing cycles. A brief summary of the present invention (e.g. the claimed ink transfer sheet and thermal transfer process) will now be provided, with a more in-depth discussion of these items being presented in the following Detailed Description of Preferred Embodiments section.
In accordance with the claimed invention, a unique ink transfer sheet and process for using the same are disclosed. To achieve the goals of the invention as discussed above, a specialized ink transfer sheet structure is initially provided. The transfer sheet is of multi-layer construction and includes a backing layer, a detachable release layer positioned on top of and adhered to the backing layer, and an ink receiving layer. The backing layer is primarily designed to provide support for the other layers in the transfer sheet while the release layer is used to adhere the ink receiving layer and printed image onto the fabric substrate. The ink receiving layer is specifically formulated to allow the adhesion and/or absorption of ink materials thereon so that a defined printed image can be effectively transferred. Further information regarding the various components and materials which can be used in connection with the multiple layers of the ink transfer sheet will be presented below.
In accordance with the claimed invention which represents a departure from the use of conventional transfer sheet structures, the ink receiving layer includes an additional ingredient which is specifically designed to produce an image-stabilizing effect and control waterfastness problems (e.g. fading) as previously discussed. Specifically, the ink receiving layer further includes at least one quaternary ammonium salt. The term xe2x80x9cquaternary ammonium saltxe2x80x9d as used herein shall be defined to involve a material which includes four separate groups (not necessarily the same) that are bonded to nitrogen in order to produce a positively-charged quaternary ammonium ion (a cation). At least one of these groups will be organic in character (e.g. will contain one or more carbon atoms). The positive charge of this cation is balanced by a selected anion. A quaternary ammonium salt produced in accordance with this general definition will have the following basic structural formula: 
In this formula, R1, R2, R3, and R4 may be selected from a wide variety of organic groups including but not limited to aliphatic and/or aromatic groups which are substituted, non-substituted, branched, or non-branched as will be discussed in greater detail below. Likewise, in accordance with the definition provided above, R1, R2, R3, and/or R4 can consist of a hydrogen group (H), provided that at least one of R1, R2, R3, and/or R4 is organic (carbon-containing) in character. In addition, Xxe2x88x92 will consist of an anion (counterion) selected from a wide variety of anions which will likewise be described further below. Effective solutions containing quaternary ammonium salts which may be used to produce the claimed ink transfer sheet by direct application thereto will have a quaternary ammonium salt concentration level of about 0.5-15% by weight. While the present invention shall not be exclusively limited to any particular quaternary ammonium salt compositions, representative and preferred quaternary ammonium salt compounds suitable for use in the claimed ink transfer sheet include but are not limited to tricaprylyl methyl ammonium chloride, ditallow dimethyl ammonium chloride, tetraoctyl ammonium bromide, and tridodecyl ammonium chloride.
To produce the completed ink transfer sheet, an untreated transfer sheet structure is first provided which includes all of the layers listed above, namely, (1) the backing layer; (2) the release layer positioned on the backing layer; and (3) the ink receiving layer on the release layer. This basic structure is a commercially available product as discussed above. However, to manufacture the claimed ink transfer sheet (e.g. the treated sheet), the selected quaternary ammonium salt is delivered (preferably in the form of an aqueous solution) directly to the upper surface of the ink receiving layer of the sheet. Application of the quaternary ammonium salt may be accomplished in any conventional manner including the use of known spraying devices or other coating systems. In accordance with the present invention, the selected quaternary ammonium salt may ultimately reside directly on top of the ink receiving layer or instead may be entirely or partially impregnated (absorbed) within the ink receiving layer. Both of these variations shall be considered equivalent to each other in form and function. The extent to which the quaternary ammonium salt will penetrate the ink receiving layer will depend on a variety of factors including the type and porosity of the materials used to manufacture the ink receiving layer as determined by preliminary pilot testing. While the claimed invention shall not be strictly limited to any particular amount of quaternary ammonium salt on the ink transfer sheet, a sufficient amount of quaternary ammonium salt will be employed in a preferred embodiment to achieve an average dry salt concentration of about 2-10 g of quaternary ammonium salt per square meter (m2) of the finished (treated) ink transfer sheet.
After production of the treated ink transfer sheet, the sheet may be used to transfer a desired printed image (either monochrome [single-color] or multi-colored) onto a selected fabric substrate (e.g. a T-shirt) in a stable, crisp, and waterfast manner. To achieve this goal, a prepared ink transfer sheet of the type described above is initially provided which again includes at least one quaternary ammonium salt as an active ingredient. Thereafter, an ink composition is also provided which contains at least one anionic (e.g. negatively-charged) coloring agent and an ink vehicle. The present invention shall not be restricted to any particular coloring agents and ink vehicles, with a wide variety of different chemical compositions being suitable for these purposes as specifically discussed in the following Detailed Description of Preferred Embodiments section. However, for the purposes of this invention, the term xe2x80x9canionic coloring agentxe2x80x9d shall be defined to encompass selected dye compositions having at least one functional chemical group which is negatively-charged and capable of reacting with the positively-charged quaternary ammonium salt in solution to produce a xe2x80x9ccomplexxe2x80x9d from the selected coloring agent. Exemplary dye/coloring agent compositions suitable for this purpose will generally include but not be limited to carboxylated and/or sulfonated dye materials known in the art, with specific examples again being provided below. Furthermore, the term xe2x80x9ccoloring agentxe2x80x9d may also encompass colorant/pigment dispersions known in the art which are made using dispersants that also include at least one functional chemical group which is capable of reacting with quaternary ammonium ions in solution to yield a complex. In a preferred embodiment, dispersants may be used which are carboxylated, sulfonated, or the like. Specific examples of color pigment dispersions which may be employed in the claimed process will be presented below.
After the desired ink composition containing at least one anionic coloring agent has been selected, it is thereafter delivered onto the ink receiving layer of the ink transfer sheet in order to form a printed image on the transfer sheet. Many different techniques may be used to accomplish ink delivery, although thermal inkjet printing methods are preferred and provide optimum results (e.g. a maximum level of clarity, simplicity, and high resolution). While thermal inkjet printing methods are of primarily interest, other inkjet systems may also be used to deliver the ink compositions of concern including piezoelectric inkjet printers, xe2x80x9ccontinuousxe2x80x9d inkjet devices, and the like. To accomplish ink delivery using thermal inkjet printing techniques, a thermal inkjet printing apparatus (printer unit) is initially provided which comprises at least one ink cartridge unit therein. The ink cartridge includes a housing and a printhead affixed to or within the housing. The printhead contains ink expulsion means for delivering ink materials from the ink cartridge, with typical ink expulsion means consisting of a plurality of thin-film resistor elements which, when electrified, heat the ink and selectively expel it from the cartridge as discussed further below. The housing of the ink cartridge further includes a supply of an ink composition therein which contains an ink vehicle and at least one anionic coloring agent as defined above. The supply of the ink composition is in fluid communication with the ink expulsion means associated with the printhead so that the printhead can selectively deliver the ink on-demand.
Delivery of the ink composition onto the ink receiving layer of the claimed ink transfer sheet is specifically accomplished in a thermal inkjet system by placing the ink transfer sheet directly within the thermal inkjet printing apparatus/printer. Thereafter, the ink expulsion means of the printhead associated with the ink cartridge is activated (e.g. energized) in order to deliver the ink composition from the ink cartridge onto the ink receiving layer of the transfer sheet to thereby form a clear and defined monochrome or multi-colored printed image on the sheet. However, as indicated above, the claimed invention shall not be exclusively limited to the use of thermal inkjet printing techniques, with other printing methods also being applicable.
Regardless of which ink delivery method is selected, once the ink composition is delivered to the ink receiving layer of the transfer sheet in a desired pattern, the anionic (e.g. negatively-charged) coloring agent in the ink composition will bind to the positively-charged quaternary ammonium salt in order to produce a chemical xe2x80x9ccomplexxe2x80x9d which is effectively fixed to the ink transfer sheet. This fixation process ultimately results in enhanced image stability on the fabric substrate which is characterized by improved waterfastness and reduced fading even after repeated machine washings.
Once the printed image has been applied to the ink receiving layer on the ink transfer sheet, the transfer sheet is placed on and against the selected fabric substrate so that the ink receiving layer (and the printed image) is in physical contact with the substrate. Many fabric materials may be used for this purpose including cotton, cotton blends, and synthetic compositions, with the present invention not being limited to any particular textile products for this purpose. Representative fabric materials which are particularly suitable for use in the claimed process will be discussed below. Thereafter, heat is applied to the ink transfer sheet while the transfer sheet is in direct contact with (positioned on) the fabric substrate. Heat is conventionally applied to the ink transfer sheet (e.g. using a standard heated platen apparatus or household iron) in an amount sufficient to cause the release layer and ink receiving layer associated therewith to soften and adhere to the fabric substrate. This is readily accomplished in accordance with the low melting point characteristics of the polymeric compounds which are typically used to manufacture the release layer. While the invention shall not be restricted to any particular temperature levels and processing times at this stage of the claimed method (which are typically determined by preliminary pilot studies), heating of the ink transfer sheet will preferably involve temperature levels of about 150-200xc2x0 C. applied for approximately 0.3-3.0 minutes while the ink transfer sheet is in direct contact with the fabric substrate. Likewise, to ensure complete transfer of the printed image to the fabric substrate, it is preferred that pressure be applied to the transfer sheet positioned on the substrate during the application of heat in an amount sufficient to facilitate complete contact between the transfer sheet and the substrate. In a representative embodiment, this pressure would typically involve about 0.05-2.0 lbs/in2 of the transfer sheet, although the exact pressure level to be used in a given situation may be determined in accordance with preliminary routine testing.
After or during the application of heat as discussed above, the backing layer is removed (e.g. by physical detachment or xe2x80x9cpeelingxe2x80x9d) from the ink transfer sheet in order to separate the release layer from the backing layer. As a result, the release layer and attached ink receiving layer (with the printed image thereon) are left on the fabric substrate. In this manner, the printed image is directly transferred to the substrate. It is important to note that the printed image (which is usually applied in a xe2x80x9creversexe2x80x9d configuration to the ink transfer sheet so that it will be properly oriented on the fabric substrate) is readily visible on the fabric substrate since both the release layer and ink receiving layer are substantially colorless (e.g. transparent). As previously indicated, the anionic (e.g. negatively-charged) coloring agent and the positively-charged quaternary ammonium salt interact to produce a precipitation/complexation reaction which stabilizes the printed image on both the ink transfer sheet and the fabric substrate. The printed image is vivid, crisp, and characterized by a high level of waterfastness (compared with ink transfer sheets that do not employ quaternary ammonium salts). As a result, the stabilized image avoids fading, color bleed, and a loss of image resolution even after repeated machine washings.
The present invention represents an advance in the art of thermal transfer printing on fabric substrates which provides numerous benefits and advantages including: (1) the rapid printing of clear, vivid, and distinct images with a minimal amount of equipment and process steps; (2) enhanced image waterfastness and fade-resistance; (3) a minimal level of complexity and required equipment which facilitates at-home use by consumers; (4) the ability to use thermal inkjet technology (or other inkjet systems) to generate high-resolution multi-color images which are characterized by improved stability levels; and (5) the ability to accomplish these goals using low-cost materials and equipment. These and other objects, features, and advantages of the invention will be discussed below in the following Brief Description of the Drawings and Detailed Description of Preferred Embodiments section.